Surely Mark has to return for Leslie's wedding at the end of the season. If he does, I reckon he and Ann will end up hooking up, and he might return as a minor recurring character?
And they want you to be surprised, that's why they haven't mentioned him in forever, and he wasn't mentioned in Ann's boxes of stuff she was selling from when she was with her old boyfriends. Or maybe she can't bare to get rid of that stuff?

I dunno, like I said, it's a theory. It'd be nice to have him back. I think he and Ben would be good buddies.

Eh, I don't miss mark at all. And I don't know about the actor's separation from the show, but since he left so suddenly, and they haven't mentioned him at all since (including, as you said, Ann's boyfriend boxes), it makes me think maybe he didn't leave on good terms. :/

I liked him but not enough for me to demand him back. I don't think he'll be back because it indeed has been too long (I was surprised to see Louis CK come back because I also thought he had been gone too long)

I heard he left because his career was taking off or something (which I don't think is true, really, unless he's in a lot of things that simply haven't been mentioned in his wiki) But I also heard he left amicably because the showrunner (?) said that they would like him back and would make it so he can/has to come back. But I guess what happened was a vicious cycle of "we don't mention him because he's gone and he's gone because we don't mention him"

I don't think his absence from the bf boxes means anything other then him not being mentioned because no one remembers him.

That seems very reasonable. Like I said, I don't know of anything about the actor or his parting. My conjecture was purely from watching the show (and, I suppose, from not seeing him in any movies or things that I would attribute to taking all his time).